Am I right in thinking the ZF in the M135i/235i can only handle 450NM? Just thinking for a tuning standpoint would it be better to go manual and get an uprated clutch? What sort of figures can you expect from these at Stage 1, Stage 2, etc?

ZF will warranty them to 450Nm, but in reality most will take far more. There's a reasonable tolerance, but that's ZF's safety margin. If you modify the car, obviously the drivetrain (engine, box and diff) warranty is void.

A Stage 1 tuning box usually gives approx 380bhp.

After that point, gains come in smaller increments - law of diminishing returns. 10bhp for a down pipe, 5bhp for an intercooler, etc. Again, all approximates.

Good to know. It's more to know if I should go manual or auto as if the auto is as good as people say then I'm fine with it. Don't want to go auto, then once the warranty is up find I'm restricted by the gearbox. Although tbf 400bhp would be nuts in something like this anyway so may well be enough